Audio scrambling system using in-band carrier

ABSTRACT

Audio signals are scrambled by single side band modulating with a modulation signal carrier having a frequency lying within the audio frequency band so that the signals are frequency translated upward. The scrambled audio signals are descrambled after broadcasting or recording/reproducing using a substantially identical modulation carrier signal to restore the original audio signals. Security is enhanced by varying the frequency of the modulation carrier signal in a pseudo random manner in response to start (ACLK) and rate (A0, A1) control signals. The control signals accompany the scrambled audio signals and are used during the descrambling process to aid in the generation of the descrambling carrier modulation signal. The frequency translation technique reduces the adverse effect of wow and flutter frequency fluctuations introduced during recording and reproduction of the audio signals (as compared to systems using frequency spectrum inversion), and also reduces the adverse affect of high frequency headroom crashing experienced in pre-emphasis broadcasting and equalizer recording applications.

BACKGROUND OF THE INVENTION

This invention relates to techniques for scrambling and descramblingaudio information signals. More particularly, this invention relates tofrequency shifting techniques for scrambling and descrambling suchsignals.

Various techniques have been employed in the past for the purpose ofinitially scrambling and subsequently descrambling audio informationsignals. One such technique is known as frequency inversion spectrumshifting, wherein the spectrum of original audio information signals isshifted by inversion so that those frequency portions originally lyingat the lower end of the audio frequency band are shifted to the upperend while those portions originally lying near the upper end of the bandare shifted to the lower end. Typically, this spectral inversion of theoriginal audio information signals is performed prior to broadcasting orrecording the signals either alone, or in combination with associatedvideo signals, and this technique is described more fully in copending,commonly assigned U.S. patent application Ser. No. 366,575, filed Jun.15, 1989 for "IMPROVED METHOD AND SYSTEM FOR SCRAMBLING AND DESCRAMBLINGAUDIO INFORMATION SIGNALS", now U.S. Pat. No. 5,058,159 issued Oct. 15,1992, the disclosure of which is hereby incorporated by reference. Themajor purpose of such scrambling is to prevent unauthorized reception orreproduction of the signals. As one commercial example, pre-recordedvideo cassettes can be rendered unintelligible by scrambling the audioinformation portion, so that only an authorized subscriber having aproper descrambling unit coupled to the television monitor/receiver canenjoy the program information by descrambling the audio portion.

A major disadvantage with known audio scrambling devices using frequencyinversion spectrum shifting techniques is the introduction of frequencyerror upon recording and reproduction of the scrambled signals, whichadversely affects the descrambling process. In particular, even if thescrambled audio signals are recorded on a cassette tape in a highquality VCR, the amount of frequency error introduced as wavering is atleast ±1% at the carrier frequency for a typical unit. This frequencyerror introduces unwanted components into the recovered signals,resulting in garbled sounds which are annoying at best andunrecognizable at worst.

As a specific example, with a frequency spectrum inversion techniqueemployed for audio signals having a bandwidth of 15 KHz, the typicalminimum carrier frequency would be 1 KHz above the upper end of theband, or 16 KHz at base band. If a 700 Hz audio signal is scrambledusing the 16 KHz carrier and then recorded on a tape in a quality VCR,the recorded signal will be 16 KHz-700 Hz±1%=15.3 KHz±153 Hz. Uponreproduction and descrambling using a carrier of the same frequency, thedescrambled signal will be 16 KHz-[15.3 KHz±153 Hz]=700 Hz±153]Hz=700Hz±21.9%. As will be appreciated by those skilled in the art, such awide amount of wavering distortion will render the descrambled audiosignals at least unpleasant if not unintelligible.

Another disadvantage encountered with frequency inversion techniques inaudio signal processing results from the pre-emphasis signal processingnormally encountered in broadcasting environments and in many recorders.More particularly, in a broadcasting application pre-emphasis is appliedin an amount of +6 db per octave beginning at about 2 KHz. For audiosignal processing, since the upper edge of the bandwidth (i.e., 15 KHz)lies in an area of high pre-emphasis, and since much of the audio energyupon frequency inversion is located at the upper edge, only relativelylow signal levels can be appropriately input to a broadcast or recordingsystem, which reduces the signal to-noise ratio by an undesirableamount.

SUMMARY OF THE INVENTION

The invention comprises a method and system for enabling scrambling oforiginal audio information signals which substantially reduces theadverse effect of frequency variations in the reproduction of thescrambled signals to produce final audio signals of quality comparableto the original unscrambled signals, and which is highly compatible withexisting pre-emphasis signal processing techniques employed inbroadcasting and recording applications.

From a method standpoint, the invention broadly comprises frequencytranslating the original spectrum of audio information signals toproduce scrambled audio information signals by generating a modulationcarrier signal having a frequency lying within the frequency spectralrange of the audio information signals, and single side band modulatingthe original information signals with the modulation carrier signal totranslate the frequency of the original audio information signal in agiven direction. Preferably, the frequency of the modulation carriersignal is varied during generation in a pseudo random fashion,particularly by sweeping the frequency of the modulation carrier signalbetween predetermined limits. The step of varying the frequency of themodulation carrier signal preferably includes the steps of initiating afrequency varying operation in response to a first control signal at arate determined by a second control signal.

The step of single side band modulating preferably includes the steps ofphase shifting the original audio information signals by 90°, phaseshifting the modulation carrier signal by 90°, double side band mixingthe original audio information signals with the 90° phase shiftedversion of the modulation carrier signal, double side band mixing the90° phase shifted version of the original audio information signals withthe modulation carrier signal, and summing the signals resulting fromthe double side band mixing steps to produce the desired scrambled audiosignals.

The scrambled signals are later descrambled by generating a descramblingmodulation carrier signal having substantially the same frequency as thescrambling modulation carrier signal, and single side band modulatingthe scrambled audio signals with the descrambling modulation carriersignal to recover the original audio signals. During descrambling, thestep of single side band modulating includes the steps of phase shiftingthe scrambled audio signals by 90°, phase shifting the descramblingmodulation carrier signal by 90°, double side band mixing the scrambledaudio signals with the descrambling modulation carrier signal, doubleside band mixing the 90° phase shifted version of the scrambled audiosignals with the 90° phase shifted version of the descrambled modulationcarrier signal, and summing the signals resulting from the double sideband mixing steps to remove the scrambling single side band signalcomponent.

From a system standpoint, the invention broadly comprises means forgenerating a modulation carrier signal at a frequency lying within theoriginal frequency spectral range of audio information signals, andprocessing means having a first input coupled to the generating meansand a second input for receiving the audio information signals forproducing scrambled audio signals resulting from the single side bandmixing of the modulation carrier signal and the audio informationsignals. The generating means preferably includes means for varying thefrequency of the modulation carrier signal in a pseudo random fashion,preferably by sweeping the frequency of the modulation carrier signalbetween predetermined limits. The generating means further preferablyincludes means responsive to a first control signal for initiating thevarying means and means responsive to a second control signal forestablishing the varying rate.

The processing means preferably includes first phase shifting means forphase shifting the modulation carrier signal by 90°, second phaseshifting means for phase shifting the audio information signals by 90°,first double side band mixing means coupled to the first phase shiftingmeans for mixing the audio information signals and the 90° phase shiftedversion of the modulation carrier signal, second double side band mixingmeans coupled to the second phase shifting means for mixing the 90°phase shifted version of the audio information signals with themodulation carrier signal, and means coupled to the first and seconddouble side band mixing means for summing the output signals therefromto provide the desired scrambled audio signals.

The descrambling system includes means for generating a descramblingmodulation carrier signal having substantially the same frequency as thescrambling modulation carrier signal, and processing means having afirst input coupled to the generating means and a second input forreceiving the scrambled audio information signals for removing thescrambled single side band signal component therefrom to recover theoriginal audio information signals. The generating means preferablyincludes means responsive to the first control signal for initiating thevarying means and means responsive to the second control signal forestablishing the varying rate.

The descrambler processing means preferably includes first phaseshifting means for phase shifting the scrambled audio signals by 90°,second phase shifting means for phase shifting the descramblingmodulation carrier signal by 90°, first double side band mixing meansfor mixing the scrambled audio signals with the descrambling modulationcarrier signal, second double side band mixing means coupled to thefirst and second phase shifting means for mixing the 90° phase shiftedversion of the scrambled audio signals with the 90° phase shiftedversion of the descrambling modulation carrier signal, and means coupledto the first and second double side band mixing means for PG,7 summingthe output signals therefrom to remove the scrambling single side bandsignal component.

The invention further comprises a system for generating the variablefrequency sine wave carrier signal in response to a start signal and arate signal, the carrier signal having frequencies lying within apredetermined spectral frequency limit, and the system including firstsettable counter means having a clock input for receiving a first clocksignal, a count input for receiving modulus value signals forestablishing the counter modulus N, and an output terminal formanifesting a binary output signal having a frequency determined by themodulus N and the first clock signal; modulus generating means having acontrol input for receiving the start signal, a clock input forreceiving a second clock signal, and an output coupled to the countinput of the first settable counter means for generating a succession ofmodulus values in response to receipt of a start signal and the secondclock signal; and means having an input coupled to the output terminalof the first settable counter means for converting the binary outputsignals therefrom to analog sine signals of variable frequency.

The modulus generating means preferably includes a second counter meanshaving an input serving the clock input of the modulus generating meansand an output, and memory means for storing the modulus values, thememory means having address inputs coupled to the output of the secondcounter means. The memory means includes a first memory device forstoring a first plurality of modulus determining values, a second memorydevice for storing a second plurality of modulus determining values, andprocessing means for receiving modulus determining values from the firstand second memory devices for generating the modulus value signals forthe first settable counter means.

The system further includes a second clock generating means, the secondclock generating means including an oscillator and a second settablecounter means having a clock input coupled to the oscillator, a countinput terminal means for receiving the rate signal, and an outputterminal means coupled to the clock input of the modulus generatingmeans, the output of the second settable counter means having afrequency determined by the value of the rate signal. The rate signalmay be either periodic, or non-periodic: if non-periodic, the ratesignal is preferably pseudo random.

The converting means preferably includes a first low pass filter meansfor smoothing the binary output signals from the first settable countermeans; means for generating a sine wave signal having a frequencydifferent from the frequency of the signals output from the first lowpass filter means; mixer means having a first input coupled to theoutput of the first low pass filter means, a second input coupled to theoutput of the sine wave signal generating means, and an output; andsecond low pass filter means having an input coupled to the output ofthe mixer means and an output for passing signals having frequencieslying within the predetermined spectral frequency limit.

For a fuller understand of the nature and advantages of the invention,reference should be made to the ensuing detailed description taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram illustrating scrambling of the audio portionof composite video signals;

FIG. 1B is a block diagram illustrating descrambling of previouslyscrambled signals;

FIG. 2 is a schematic diagram illustrating the modulation circuitry forscrambling the original audio signals;

FIG. 3A is a frequency spectrum diagram illustrating the relationshipbetween the modulation carrier and the original audio pass band;

FIG. 3B is a diagram similar to FIG. 3A illustrating the frequencytranslation effected according to the invention;

FIG. 4 is a schematic diagram illustrating the demodulation circuitryfor descrambling the scrambled audio signals;

FIG. 5 is a schematic diagram illustrating the modulation carrier signalgenerator used in the scrambling/descrambling units;

FIGS. 6A-6D are timing diagrams illustrating the modulation carriersignal generation process;

FIG. 7 is a logic diagram illustrating the K counter of the generator ofFIG. 5;

FIG. 8 is a logic diagram illustrating the divide by N counter of thegenerator of FIG. 5;

FIG. 9 is a circuit diagram of a single stage of the wide band phaseshifter used in the modulation circuitry of FIG. 2;

FIG. 10 is a circuit diagram illustrating the low pass filters used inthe generator of FIG. 5;

FIG. 11 is a circuit diagram showing the double side band mixer used inthe circuitry of FIG. 2; and

FIG. 12 is a schematic diagram illustrating an alternate embodiment ofthe modulus generator for the divide by N counter of FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to the drawings, FIG. 1A illustrates use of the invention toscramble the audio portion of video program signals prior to recordingboth the video and audio portions onto a video cassette. As seen in thisFig., the video program information signals, including coded controlsignals termed ACLK, A0, and A1 (described below) are coupled directlyto the video input of a conventional video cassette recorder 12. Theaudio portion, on the other hand, is coupled to the audio input of thescrambler unit 14 in which the audio signals are scrambled in the mannerdescribed below. The control signals ACLK, A0 and A1 are also coupled toa control input portion of scrambler unit 14. The scrambled audio outputsignals from scrambling unit 14 are then coupled to the audio inputterminal of video cassette recorder unit 12. FIG. 1B illustrates theplayback mode for the video and scrambled audio signals prepared in theFIG. 1A system. As seen in FIG. 1B, the video program informationappearing at the video output terminal of the VCR unit 12 is coupleddirectly through a descrambler unit 16 to the associated follow-onelectronics (i.e., the television monitor 15). The scrambled audiosignals, in contrast, are coupled to the input of the descrambler unit16. Also, as suggested by the broken line and legends within unit 16,the ACLK, A0 and A1 signal portions of the video output from VCR unit 12are separated and coupled to the control input portion of descramblerunit 16 for descrambling in the manner described below. The output fromthe descrambler unit 16 is the audio program portion, now unscrambled,which is amplified by amplifier 17 and coupled to speaker 18.

The scrambling technique employed in the preferred embodiment of theinvention is frequency shifting the original audio information signalsby upper single side band frequency translation; the descramblingtechnique employed is lower single side band frequency translation torestore the original audio band frequencies. With reference to thescrambling technique, the original audio information signals areprocessed by single side band modulation using a modulation carrierhaving a frequency lying within the original audio information signalband so that all portions of the original audio information signals areshifted up by an amount dependent upon the frequency of the modulationcarrier. In addition, the frequency of the modulation carrier signal isvaried in a unique manner so that the absolute value of a givenfrequency in the original audio information signal spectrum changesduring scrambling in an unpredictable manner. As a result, the scrambledaudio signals, if applied to conventional audio processing circuitry ofa receiver/monitor, are so garbled as to be unusable.

The aperiodic variation of the modulation carrier frequency is initiatedby an aperiodic timing signal ACLK, which is generated in a pseudorandom fashion and which initiates a monotonic sweeping of themodulation carrier from a starting frequency to an ending frequency. Therate at which the frequency is varied is determined by the A0 and A1binary rate control signals, which are generated in accordance with anysuitable scrambling scheme. Thus, the ACLK signal controls the start ofthe modulation carrier frequency variation, while A0 and A1 control therate of change of the modulation carrier frequency. The ACLK, A0 and A1signals are encoded in the vertical interval of the video frames andrecorded along with the scrambled audio signals for later reproductionand use during the descrambling operation.

FIG. 2 illustrates that portion of the scrambler unit 14 of FIG. 1A inwhich the original audio frequency signals are shifted upwards infrequency by single side band modulation using a modulation carriersignal having a frequency lying within the original audio signalfrequency spectrum. As seen in this Fig., the original audio programinformation signals having frequencies ωb (=2πf) lying within a typicalaudio frequency spectrum of 50 Hz to 15 KHz are coupled to the input ofa first wide band phase shifter 21 which provides as output signals theoriginal input signals and a phase shifted version of the original inputsignals, with the phase difference being a constant 90°. The 0° outputfrom phase shifter 21 is coupled as a first input to a first double sideband mixer 23, while the 90° phase shifted version of the original inputsignals is coupled as a first input of a second double side band mixer24. A modulation carrier signal source 25 preferably of the typeillustrated in FIGS. 5-8, 10 and 11 generate a modulation carrier signalof frequencies ωc lying in the preferred embodiment) from about 1.4 KHzto about 3 KHz. The modulation carrier signal output from source 25 iscoupled to a second wide band phase shifter 26 identical to wide bandphase shifter 21, the first stage of which is illustrated in FIG. 9. The0° output from wide band phase band mixer 24, while the 90° version ofthe modulation carrier signal from wide band phase shifter 26 is coupledas the second input to double side band mixer 23. The modulation productoutputs from mixers 23, 24 are coupled through a summing amplifier 28,the output of which comprises the scrambled version of the originalaudio input signals, which are coupled to the VCR unit 12 shown in FIG.1A. The double side band mixers 23, 24 preferably have the configurationshown in FIG. 11.

Since the output of mixer 23 is [sin ωbt]×[cos ωct], and the output ofmixer 24 is [cos ωbt]×[sin ωct], where ωb are the frequencies in theoriginal audio band and ωc are the carrier frequencies, the output ofsumming amplifier 28 is the sum of the two product terms which simplyequals sin [ωb+ωc]t. It should be noted that the use of two double sideband mixers to generate the upper side band frequency translation isnecessary due to the fact that the modulation carrier frequency lieswithin the audio pass band. This is illustrated in FIG. 3A, which is aplot of signal amplitude v versus frequency f for the audio pass bandshowing the cut-off at about 15 KHz and a single carrier frequency ofabout 1.5 KHz. If only one double side band mixer were used, the lowerside band would interfere with the upper side band because thedifference frequency (i.e., the lower side band) is beat by a frequencylower than the upper end of the audio band. This mixing produces lowerside band frequencies from D.C. to 13.5 KHz. Thus, from 1.5 KHz to 13.5KHz both the upper and lower side bands mix, and this normally adverseeffect is avoided by the use of the two double side band mixers 23, 24.

FIG. 3B illustrates the translation effect on the original audio programfrequencies of the modulation circuitry of FIG. 2. The original audiosignals have been translated upperwardly by an amount equal to thecarrier frequency (i.e., the entire spectrum has been shifted upward byan amount ωc). As will be understood by those skilled in the art, if ωcvaries in value with time, the real time effect of the modulation is anupward shift from base band by an amount equal to the instantaneousvalue of ωc.

FIG. 4 illustrates the demodulation circuitry used in the descramblerunit 16 of FIG. 1B. As seen in FIG. 4, descrambling of the scrambledaudio supplied from the VCR 12 at frequency ωb' is accomplished byessentially identical circuitry configured as a lower single side bandmixer. In particular, the 0° output from the second wide band phaseshifter 26 is coupled as the second input of double side band mixer 23,while the 90° output from second wide band phase shifter 26 is coupledas the second input of double side band mixer 24 (compare FIG. 2 inwhich the outputs of the second wide band phase shifter 26 are coupledin reverse order to mixers 23, 24). The output of summing amplifier 28in the descrambler unit of FIG. 4 is [sin ωb't]×[sin ωct]+[cosωb't]×[cos ωct]=cos ∂ωb'-ωc]t where ωb'=ωb+ωc. Thus, the output=cos[ωb+ωc-ωc]t=cos ωbt. The use of a single side band mixer is essentialfor descrambling the scrambled audio information. If a mixer, such as adouble side band mixer, is used, the descrambled audio would result inlarge amounts of distortion due to the fact that the lower and upperside bands interfere within the audio base band spectrum. Only the useof a single side band mixer will yield an undistorted descrambledoutput.

It should further be noted that the modulation and demodulation carriersignals should be as close as possible to constant amplitude pure sinewave sources. If either carrier is not a pure sine wave, intermodulationdistortion will occur during descrambling. For example, assume that thecarrier ωc is 1 KHz with 10% third harmonic distortion at 3 KHz. If theaudio program signal has a frequency of 500 Hz, then the output of thescrambler 14 will be 1500 Hz (1 KHz+500 Hz) and 3500 Hz (3 KHz+500 Hz).In this example, even if the descrambling modulation frequency is a puresine wave at 1 KHz, the descrambled tones will be 1500 Hz-1 KHz=500 Hzand 3500 Hz-1 KHz=2500 Hz, the latter comprising an undesired distortionproduct. This distortion is exacerbated if the descrambling modulationcarrier is the same as the scrambling modulation carrier, since thedescrambled output from summing amplifier 28 will now be 1500 Hz-1 KHz,1500 Hz-3 KHz, 3500 Hz-1 KHz, and 3500 Hz-3 KHz, which yields 500 Hz,1500 Hz, 2500 Hz, and 500 Hz. In this case, the signals at 1500 Hz and2500 Hz are the extraneous distortion products. As a guideline, forpurposes of this invention a pure sine wave is defined as one havingless than 0.1 % harmonic distortion (i.e., second or higher orderharmonic components of amplitude less than 0.1 % of the carrierfrequency). In addition, the amplitude of the modulation carrier signalshould be substantially constant.

The improvement afforded by the simple translation modulation systemdescribed above over an inverted side band modulation system may now beexplored. For the system described above, and assuming a modulationcarrier frequency of 500 Hz and an audio signal of 700 Hz, the output ofthe scrambler unit 14 (FIG. 1A) will be 500 Hz+700 Hz=1200 Hz. If thissignal is recorded on a VCR which has ±1% variation, the playback signalwill be 1200 Hz±1%=1200 Hz±12 Hz. The descrambled output will then be1200 Hz±12 Hz-500 Hz=700 Hz±12 Hz=700 Hz±1.7%.

With an inverted side band system operating on an audio pass band havinga 15 KHz bandwidth, the carrier frequency must be at least 16 KHz to bemarginally outside the upper bandwidth. For the same audio signal as inthe example noted above, the scrambled output would be 1600 Hz-700Hz=15.3 KHz. When recorded and played back on the same VCR with a ±1%speed variation, the scrambled tone during playback is 15.3 KHz±1%=15.3KHz±153 Hz. The corresponding descrambled output is 1600 KHz-15.3KHz±153 Hz=700 Hz±153 Hz=700 Hz±21.9%. The improvement in the frequencyVariation distortion performance afforded by the invention is apparent.

The harmonic distortion performance is also enhanced according to theinvention over the inverted side band system due to the high frequencyhead room crashing frequently encountered in the inverted side bandsystem. In particular, the signal level of an audio signal to bebroadcast or recorded is typically pre-emphasized prior to recording orbroadcasting beyond about 2000 Hz in the amount of about 6 db peroctave. As a consequence, at 15 KHz the maximum permissible level of ainput signal into a broadcast system is about -17 db. Similarperformance is obtained with analog tape recorders. With an invertedside band system, the frequency spectrum of the original signals isinverted: i.e., those frequency portions originally lying at the lowerend of the audio frequency band are shifted to the upper end while thoseportions originally lying at the upper end of the band are shifted tothe lower end. With such a system, since most of the energy is at about15 KHz, the inverted side band method only allows about -17 db of inputover the entire audio bandwidth into a broadcast system. With thefrequency translation technique according to the invention, however, themaximum input level is still about 0 db (i.e., the same level as that ina system without inversion), since the translated spectrum is almostidentical with the spectrum of the base band. For example, for voicefrequencies (typically in the range from 300 Hz-3 KHz), a frequencytranslation of 1 KHz according to the invention yields a spectrum of1300 Hz to 4 KHz, while the prior art frequency inversion techniqueyields a spectrum of 14.7 KHz to 12 KHz. Thus, using the prior artinverted side band technique, with a broadcast or analog tape decksystem the input will have to be at -17 db whereas with the translationsystem according to the invention the input can be at -6 db.Consequently, the translation system according to the invention improvesthe signal-to-noise ratio by as much as 11 db.

As noted above, the reason for scrambling the audio signals is to limitenjoyment of the program information represented by the audio signals toauthorized subscribers. The security of the simple translation systemjust described is greatly enhanced by varying the frequency of themodulation carrier signal. FIGS. 5-8, 10 and 11 illustrate the preferredembodiment for accomplishing pseudo random carrier frequency variationaccording to the invention in both the scrambler unit 14 and thedescrambler unit 16. With reference to FIG. 5, a 15 bit counter 50 has astart input terminal 51 to which the ACLK control signal is coupled anda clock input terminal 52 to which the counter clock is supplied. Thecounter clock is supplied from a counter 53 termed the ÷ K counterhaving a pair of preset inputs 54, 55 to which the A0, A1 rate controlsignals are supplied. The clock input terminal 56 of counter 53 issupplied with a 1 MHz clock signal from a source 57. The state outputsof 15 bit counter 50 are coupled as address inputs to an EPROM memoryunit 60. Counter 50 is configured to continuously count down afterstarting from a maximum value to a minimum value, automatically reset tothe maximum count state, and await the next start pulse ACLK. The 8 bitoutput from EPROM 60 is coupled to the preset inputs of a ÷ by N counter62. Counter 62 has a clock input 63 which is supplied with a 15 MHzclock signal from a crystal oscillator source 64. The output of ÷ by Ncounter 62, in the preferred embodiment, is a 6 KHz-7.6 KHz square wave,which is coupled to the input of a first low pass filter 66, the outputof which is coupled to a first input of a mixer 67. The other input tomixer 67 is the output of a 9 KHz sine wave source 68. The output ofmixer 67 is coupled to the input of a low pass filter 69, which issubstantially identical in structure and function to the filter 66. Theoutput of low pass filter 69 is a variable frequency sine wave whosefrequency ranges from 1400 Hz to 3000 Hz. The configuration of low passfilters 66, 69 in the preferred embodiment is illustrated in FIG. 10.

In operation, counter 50 is enabled to begin counting from an initialcount by the leading edge of an ACLK signal (FIG. 6B). The rate at whichcounter 50 is decremented is determined by the clock input on terminal52 supplied by ÷ by K counter 53. Although the clock input present oninput terminal 56 to the ÷ by K counter 53 is a constant frequency (1MHz) clock signal from source 57, the value of K and thus the frequencyof the output clock from K counter 53 is dependent upon the values ofA0, A1 on the control input terminals 54, 55. ACLK, A0 and A1 signalsare all generated initially at the scrambling site and are conveyedalong with the scrambled audio information to the descrambling site. Inthe video system illustrated in FIGS. 1A and 1B, control signals ACLK,A0 and A1 are all embedded in appropriate portions of the verticalinterval of the video signals in any appropriate fashion.

As counter 50 is clocked by the clock signal from ÷ by K counter 53, thevalues stored in EPROM 60 are read out at a rate determined by the rateat which counter 50 is being clocked. In the preferred embodiment, EPROM60 is completely read within a range of about 1.5 to 10 seconds. Foreach value output from EPROM 60, the ÷ by N counter 62 generates asquare wave at a specific frequency within the range of 6 KHz-7.6 KHz,depending on the value of N (the counter modulus) specified by theoutput of EPROM 60. The square wave output from counter 62 is smoothedin low pass filter 66 to provide a relatively pure sine wave whoseinstantaneous frequency is in the range from 6 KHz-7.6 KHz. This signalis mixed in mixer 67 with the 9 KHz source 68, and the output from themixer 67 is low pass filtered by filter 69 to provide a constantamplitude pure sine wave whose instantaneous frequency lies within therange of 1.4 KHz-3.0 KHz.

The operation of the FIG. 5 circuit is illustrated in FIGS. 6A-6D. Inparticular, FIG. 6A illustrates the state of the counter 50 output,while FIGS. 6A and 6B show the relationship between the ACLK signal andthe start of the decrementing of counter 50. Note that the distancebetween successive leading edges of the ACLK signal varies in a randomor pseudo random fashion and is not constant. Thus, for the first ACLKleading edge, counter 50 commences counting from a maximum count valuein a linear fashion down to the minimum value, automatically resets tothe maximum count and waits for the leading edge of the next ACLK pulsesignal. When this occurs, the counter again commences counting down fromthe maximum to the minimum value, resets to the maximum state, etc.

The slope of the counter 50 output trace and thus the countdown rate isdetermined by the value of A0 and A1 when counter 50 is started by theleading edge of ACLK signal. The relationship between the four possibleA0, A1 values (for the two bit control signal version illustrated) andthe output frequency of the ÷ by K counter 53 is illustrated in FIG. 6C.As seen in this Fig., which plots frequency of the counter 50 inputclock versus time, the first pair of values of A0=0, A1=1 provides afirst count frequency producing the counter 50 countdown slope depictedin the region of FIG. 6A after the leading edge of the first ACLK pulse.Similarly, the second pair of values of A0=1, A1=1 provides a differentcounter 50 clock frequency (the highest frequency in the example shown)producing the counter 50 countdown slope in the region after the leadingedge of the second ACLK pulse. Similarly, A0=0, A1=0 provides the lowestcounter 50 clock frequency and the slope depicted in the region afterthe leading edge of the third ACLK pulse.

FIG. 6D shows the effect on the carrier signal output from low passfilter 69 of the combined ACLK, A0 and A1 signals. During the countperiod commencing with the leading edge of the first ACLK pulse thefrequency of the sine wave output from filter 69 sweeps between theminimum and maximum frequencies (1400 Hz-3000 Hz) at a first linearrate, starting at some initial value determined by the first modulus Noutput value from EPROM 60. After the counter 50 has counted down to theminimum value and is automatically reset, the output frequencystabilizes at the initial value during interim period designated X₁,which is the waiting period between the end of the countdown of counter50 and the arrival of the leading edge of the next ACLK pulse. Uponarrival of the second ACLK pulse, counter 50 is decremented at themaximum rate (specified by A0=1, A1=1) and the frequency of the sinewave output from filter 69 varies between the limits in a linear fashionswept at a higher frequency. When counter 50 counts down and is reset,the sine wave output remains at the constant mid-value depicted duringinterim period X₂ until the arrival of the leading edge of the next ACLKpulse. The operation continues as already described for each successiveappearance of the A0, A1 values and the leading edge of the ACLK pulse.

In the scrambling operation, the ACLK, A0, and A1 signals are allprovided to scramble unit 14 from a suitable source. In the FIG. 6A-6Dembodiment, the repetition rate of successive ACLK signals is preferablyvaried in an unpredictable manner, using any known pseudo randomcounting device; while the values of A0, A1 are also pseudo randomlygenerated. In addition, the spacing between successive ACLK pulses andthe rate values A0, A1 are selected such that counter 50 is permitted todecrement to the minimum value before the start of the next counter 50cycle, so that the frequency variation is not subject to a suddendiscontinuity which could result from a premature resetting of counter50 and which is undesirable.

In the descrambling operation, the ACLK, A0, A1 signals must be providedto the descrambler unit 16 along with the scrambled audio signals sothat the descrambling modulation carrier signal is generated in such amanner as to match the scrambling modulation carrier signal. Thisprovision of the control signals can be accomplished in any suitablemanner known to those skilled in the art. In the example shown in FIGS.1A and 1B utilizing associated video signals, the ACLK, A0 and A1signals can be inserted into the vertical blanking intervals of selectedvideo fields and detected using conventional detection circuitry.

Other combinations of ACLK, A0 and A1 start and rate signals may beemployed. For example, the repetition rate of ACLK may be fixed at someperiodic value, and the values of A0, A1 or both may be changed randomlyduring the time period between successive ACLK pulses. The effect is tovary the clock 2 input on terminal 52 of counter 50 in a pseudo randommanner, which causes a similar effect at the output of counter 50. Foroptimum operation, the relative values of the ACLK repetition rate andthe rates specified by A0, A1 should be selected so that counter 50 isdecremented to the final value (by the clock 2 signal) prior to reset bythe succeeding ACLK pulse to avoid a sharp discontinuity in thefrequency of the output of counter 62.

FIG. 9 illustrates one stage of a pair of phase shifters providing twoconstant amplitude phase shifters yielding a pair of net outputs of 0°and 90°. All of the R resistances have the same value. However, thevalues of R₁, C₁ and R₂, C₂ are selected to provide the 90° differencebetween the two outputs VOUT₁ and VOUT₂. To increase the range of theupper and lower frequencies, pairs of phase shifter sections illustratedin FIG. 9 are added in cascade to construct phase shifters 21, 26 of thescrambler and descrambler units.

With reference to FIG. 11, which illustrates mixers 23, 24 of thescrambler and descrambler units, resistances Re₁ provide local feedbackto linearize the Q₁ and Q₂ collector current versus the input voltageIN₁ at the bases of Q₁, Q₂ Diodes D₁ and D₂ predistort the drivingvoltage into Q₃ -Q₆ such that the transfer function from IN₁ to theoutput voltage at RL₆ is linear. Voltage IN₂ is linearized for Q₇ and Q₈collector current via both resistances Re₂. Due to the push/pullarrangement of the collector currents of Q₇ and Q₈ modulating theamplifier pairs of Q₃, Q₄ and Q₅, Q₆, a linear relationship is providedbetween IN₂ and the output signal. In general, the output signal equalsK×IN₁ ×IN₂, where K is a constant.

FIG. 12 illustrates an alternate embodiment of the invention whichprovides enhanced security to the process for generating the modulus Nfrequency specifying signals for counter 62. As seen in FIG. 12, thesingle EPROM memory unit 60 is replaced by a pair of EPROM memory units601, 602, each having multi-bit address input terminals coupled to theoutput of counter 50. Each EPROM memory unit 601, 602 stores a pluralityof 8 bit modulus determining values which are individually addressed bythe output of counter 50 and coupled to the paired inputs of anarithmetic logic unit 603. Also coupled to the input of arithmetic logicunit 603 are a pair of numerical coefficient values designated a, b,which are multi-bit (i.e., 4 bit) randomly generated numbers. The outputof ALU 603 is coupled to the address input of divide by N counter 62.The alternate embodiment shown in FIG. 12 adds enhanced security bygenerating each modulus specifying value N from a pair of multi-bit (8bit) values stored in the memory units 601, 602 and the two randomnumbers a, b, which are used in any desired algorithmic manner. Forexample, the output from ALU 603 may be simply the linear combinationaX+bY, where X and Y are the output values from memory devices 601, 602;similarly, the coefficient a, b may additionally be used as acombinatorial divisor for the values X, Y, or in any other suitablefashion.

As will now be apparent, the invention provides scrambling anddescrambling of audio information signals which results in the recoveryof the original audio information signals in a faithful manner. Thesignals are recovered with minimal fluctuations in phase and frequencyintroduced by the electromechanical recording apparatus used to recordand reproduce the signals; and the recovered signals are relativelyunaffected (as compared to frequency inversion scrambling) by thelimited high frequency head room found in broadcast pre-emphasis andrecord equalization systems. At the same time, the audio signals arescrambled in a very secure fashion by use of the pseudo random varyingmodulation carrier, while requiring a substantially narrower bandwidththan other audio scrambling techniques, in particular, digitalscrambling systems. Further, since the entire scrambling anddescrambling signal processing occurs in the analog domain, relativelylow distortion is introduced to original audio input signals having arelatively low level, as opposed to digital systems which can completelycorrupt low level input signals.

While the above provides a full and complete disclosure of the preferredembodiment of the invention, various modifications, alternateconstructions and equivalents will appear to those skilled in the art.For example, while specific frequencies have been illustrated for themodulation carrier signal, other values may be more suitable in a givenapplication. Also, the security afforded by the pseudo random varyingmodulation carrier frequency may be enhanced by adding passwords,additional control signals supplemental to A0, A1 and other techniquesknown to those skilled in the art. Therefore, the above descriptions andillustrations should not be construed as limiting the invention which isdefined by the appended claims.

What is claimed is:
 1. A method of scrambling audio information signalshaving frequency components lying within an original frequency spectralrange of about 50 Hz to about 15 kHz, said method comprising the stepsof:(a) generating a modulation carrier signal at a frequency lyingwithin the original frequency spectral range, and (b) single side bandmodulating the original audio information signals with the modulationcarrier signal from step (a) to translate the frequency of said originalaudio information signals in a given direction, the resulting signalshaving a frequency greater than the frequency of the carrier signal. 2.The method of claim 1 wherein said step (a) of generating includes thestep of varying the frequency of the modulation carrier signal.
 3. Themethod of claim 2 wherein said step of varying includes the step ofcontinuously sweeping the frequency of the modulation carrier signalbetween predetermined limits.
 4. The method of claim 2 wherein said stepof varying is performed in a pseudo random fashion.
 5. The method ofclaim 2 wherein said step of varying includes the steps of initiating afrequency varying operation in response to a first control signal (A0,A1) at a rate determined by a second control signal (A0, A1).
 6. Themethod of claim 1 wherein said step (b) of single side band modulatingincludes the steps of phase shifting the original audio informationsignals by 90°, phase shifting the modulation carrier signal by 90°,double side band mixing the original audio information signals with the90° phase shifted version of the modulation carrier signal, double sideband mixing the 90° phase shifted version of the original audioinformation signals with the modulation carrier signal, and summing thesignals resulting from the double side band mixing steps to produce thedesired scrambled audio signals.
 7. A method of descrambling audioinformation signals previously scrambled by generating a scramblingmodulation carrier signal at a frequency lying within the frequencyspectral range of original audio information signals of about 50 Hz toabout 15 kHz and single side band modulating the original audioinformation signals with the modulation carrier signal to producescrambled audio signals containing a frequency translated version of theoriginal audio information signals, said descrambling method comprisingthe steps of:(a) generating a descrambling modulation carrier signalhaving substantially the same frequency as the scrambling modulationcarrier signal; and (b) single side band modulating the scrambled audiosignals with the descrambling modulation carrier signal to remove thescrambling single side band signal component and recover the originalaudio information signals, said recovered audio information signalshaving a maximum frequency greater than the frequency of the carriersignal.
 8. The method of claim 7 wherein the scrambled audio signalshave a variable frequency component contributed by the scramblingmodulation carrier signal, and wherein said step (a) of generatingincludes the step of varying the frequency of the descramblingmodulation carrier signal in a substantially identical manner as thevariation in the scrambled audio signals.
 9. The method of claim 8wherein said variable frequency component of said scrambled audiosignals comprises a swept frequency, and wherein said step of varyingincludes the step of continuously sweeping the frequency of saiddescrambling modulation carrier signal in substantially the same manner.10. The method of claim 9 wherein said swept frequency is pseudo random,and wherein said step of sweeping is performed in substantially the samepseudo random manner.
 11. The method of claim 8 wherein said scrambledaudio signals include associated first control signals (A0, A1)specifying the start of a frequency varying operation and secondassociated control signals (A0, A1) specifying the frequency varyingrate; and wherein said step of varying includes the steps of initiatinga frequency varying operation in response to said first control signalsat a rate determined by said second control signals.
 12. The method ofclaim 7 wherein said step (b) of single side band modulating includesthe steps of phase shifting the scrambled audio signals by 90°, phaseshifting the descrambling modulation carrier signal by 90°, double sideband mixing the scrambled audio signals with the descrambling modulationcarrier signal, double side band mixing the 90° phase shifted version ofthe scrambled audio signals with the 90° phase shifted version of thedescrambling modulation carrier signal, and summing the signalsresulting from the double side band mixing steps to remove thescrambling single side band signal component.
 13. A system forscrambling audio information signals having frequency components lyingwithin an original frequency spectral range of about 50 Hz to about 15kHz, said system comprising:means for generating a modulation carrierfrequency lying within the original frequency spectral range; and singleside band processing means having a first input coupled to saidgenerating means and a second input for receiving said audio informationsignals for producing scrambled audio signals resulting from the singleside band mixing of said modulation carrier signal and said audioinformation signals, said scrambled audio signals having a frequencygreater than the frequency of said carrier signal.
 14. The system ofclaim 13 wherein said generating means includes means for varying thefrequency of said modulation carrier signal.
 15. The system of claim 14wherein said varying means includes means for continuously sweeping thefrequency of said modulation carrier signal between predeterminedlimits.
 16. The invention of claim 14 wherein said generating meansincludes means responsive to a first control signal (A0, A1) forinitiating said varying means and means responsive to a second controlsignal (A0, A1) for establishing the varying rate.
 17. The invention ofclaim 13 wherein said processing means includes first phase shiftingmeans for phase shifting said modulation carrier signal by 90°, secondphase shifting means for phase shifting said audio information signalsby 90°, first double side band mixing means coupled to said first phaseshifting means for mixing said audio information signals and the 90°phase shifted version of the modulation carrier signal, second doubleside band mixing means coupled to said second phase shifting means formixing the 90° phase shifted version of the audio information signalswith the modulation carrier signal, and means coupled to said first andsecond double side band mixing means for summing the output signalstherefrom to provide the desired scrambled audio signals.
 18. A systemfor descrambling audio information signals previously scrambled bygenerating a scrambling modulation carrier signal at a frequency lyingwithin the frequency spectral range of original audio informationsignals of about 50 Hz to about 15 kHz and single side band modulatingthe original audio information signals with the modulation carriersignal to produce scrambled audio signals containing a frequencytranslated version of the original audio information signals, saidsystem comprising:means for generating a descrambling modulation carriersignal having substantially the same frequency as the scramblingmodulation carrier signal; and single side band processing means havinga first input coupled to said generating means and a second input forreceiving said scrambled audio signals for removing the scramblingsingle side band signal component therefrom to recover said originalaudio information signals, said recovered audio information signalshaving a maximum frequency greater than the frequency of said carriersignal.
 19. The system of claim 18 wherein the scrambled audio signalshave a variable frequency component contributed by the scramblingmodulation carrier signal, and wherein said generating means includesmeans for varying the frequency of the descrambling modulation carriersignal in a substantially identical manner to the variations in thescrambled audio signals.
 20. The system of claim 19 wherein saidvariable frequency component of said scrambled audio signals comprises aswept frequency, and wherein said varying means includes means forcontinuously sweeping the frequency of said descrambling modulationcarrier signal in substantially the same manner.
 21. The system of claim19 wherein said scrambled audio signals include associated first controlsignals specifying the start of frequency varying operations and secondassociated control signals specifying the frequency varying rate; andwherein said generating means includes means responsive to said firstcontrol signals for initiating said varying means and means responsiveto said second control signals for establishing the varying rate. 22.The system of claim 18 wherein said processing means includes firstphase shifting means for phase shifting the scrambled audio signals by90°, second phase shifting means for phase shifting the descramblingmodulation carrier signal by 90°, first double side band mixing meansfor mixing the scrambled audio signals with the descrambling modulationcarrier signal, second double side band mixing means coupled to saidfirst and second phase shifting means for mixing the 90° phase shiftedversion of the scrambled audio signals with the 90° phase shiftedversion of the descrambling modulation carrier signal, and means coupledto said first and second double side band mixing means for summing theoutput signals therefrom to remove the scrambling single side bandsignal component.
 23. A method of scrambling audio information signalshaving frequency components lying within an original frequency spectralrange, said method comprising the steps of:(a) applying frequencypre-emphasis to the audio information signals; (b) generating amodulation carrier signal at a frequency less than the maximum of theaudio information signals; and (c) signal side band modulating the audioinformation signals with the modulation carrier signal from step (b) totranslate the frequency of said audio information signals in a givendirection, without any filtering to remove the suppressed side band. 24.A system for scrambling audio information signals having frequencycomponents lying within an original frequency spectral range, saidsystem comprising:means for generating a modulation carrier signal at afrequency less than the maximum of the audio information signals; and asingle side band processor having only a single mixing stage and havinga first input coupled to said generating means and a second input forreceiving said audio information signals for producing scrambled audiosignals resulting from the single side band mixing of said modulationcarrier signal and said audio information signals, without any filterfor removing the suppressed side band.
 25. A method of scrambling audioinformation signals having frequency components lying within an originalfrequency spectral range of about 50 Hz to about 15 kHz, said methodcomprising the steps of:(a) generating a modulation carrier signal at afrequency lying within the original frequency spectral range, and (b)single band modulating the original audio information signals with themodulation carrier signal from step (a) to translate the frequency ofsaid original audio information signals in a given direction, whereinsaid step (a) of generating includes the step of varying the frequencyof the modulation carrier signal, and wherein said step of varyingincludes the steps of initiating a frequency varying operation inresponse to a first control signal at a rate determined by a secondcontrol signal.
 26. A method of scrambling audio information signalshaving frequency components lying within an original frequency spectralrange of about 50 Hz to about 15 kHz, said method comprising the stepsof:(a) generating a modulation carrier signal at a frequency lyingwithin the original frequency spectral range, and (b) single side bandmodulating the original audio information signals with the modulationcarrier signal from step (a) to translate the frequency of said originalaudio information signals in a given direction; wherein said step (b) ofsingle side band modulating includes the steps of phase shifting theoriginal audio information signals by 90°, phase shifting the modulationcarrier signal by 90° phase shifted version of the modulation carriersignal, double side band mixing the 90° phase shifted version of theoriginal audio information signals with the modulation carrier signal,and summing the signals resulting from the double side band mixing stepsto produce the desired scrambled audio signals.
 27. A method ofscrambling audio information signals previously scrambled by generatinga scrambling modulation carrier signal at a frequency lying within thefrequency spectral range of original audio information signals of about50 Hz to about 15 kHz and single side band modulating the original audioinformation signals with the modulating carrier signal to producescrambled audio signals containing a frequency translated version of theoriginal audio information signals, said descrambling method comprisingthe steps of:(a) generating a descrambling modulation carrier signalhaving substantially the same frequency as the scrambling modulationcarrier signal; and (b) single side band modulating the scrambled audiosignals with the descrambling modulation carrier signal to remove thescrambling single side band signal component and recover the originalaudio information signals; wherein the scrambled audio signals have avariable frequency component contributed by the scrambling modulationcarrier signal, and wherein said step (a) of generating includes thestep of varying the frequency of the descrambling modulation carriersignal in a substantially identical manner as the variation in thescrambled audio signals; and wherein said scrambled audio signalsinclude associated first control signals specifying the start of afrequency varying operation and second associated control signalsspecifying the frequency varying rate; and wherein said step of varyingincludes the steps of initiating a frequency varying operation inresponse to said first control signals at a rate determined by saidsecond control signals.
 28. A method of scrambling audio informationsignals previously scrambled by generating a scrambling modulationcarrier signal at a frequency lying within the frequency spectral rangeof original audio information signals of about 50 Hz to about 15 kHz andsingle side band modulating the original audio information signals withthe modulating carrier signal to produce scrambled audio signalscontaining a frequency translated version of the original audioinformation signals, said descrambling method comprising the stepsof:(a) generating a descrambling modulation carrier signal havingsubstantially the same frequency as the scrambling modulation carriersignal; and (b) single side band modulating the scrambled audio signalswith the descrambling modulation carrier signal to remove the scramblingside band signal component and recover the original audio informationsignals; and wherein said step (b) of single side band modulatingincludes the steps of phase shifting the scrambled audio signals by 90°,phase shifting the descrambling modulation carrier signal by 90°, doubleside band mixing the scrambled audio signals with the descramblingmodulation carrier signal, double side band mixing the 90° phase shiftedversion of the descrambling modulation carrier signal, and summing thesignals resulting from the double side band mixing steps to remove thescrambling single side band signal component.
 29. A system forscrambling audio information signals having frequency components lyingwithin an original frequency spectral range of about 50 Hz to abut 15kHz, said system comprising:means for generating a modulation carriersignal at a frequency lying within the original frequency spectralrange; and single side band processing means having a first inputcoupled to said generating means and a second input for receiving saidscrambled audio information signals for producing scrambled audiosignals resulting from the single side band mixing of said modulationcarrier signal and said audio information signals; wherein saidgenerating means includes means for varying the frequency of saidmodulation carrier signal, and wherein said generating means includesmeans responsive to a first control signal for initiating said varyingmeans and means responsive to a second control signal for establishingthe varying rate.
 30. A system for scrambling audio information signalshaving frequency components lying within an original frequency spectralrange of about 50 Hz to about 15 kHz, said system comprising:means forgenerating a modulation carrier signal at a frequency lying within theoriginal frequency spectral range; and single side band processing meanshaving a first input coupled to said generating means and a second inputfor receiving said audio information signals for producing scrambledaudio signals resulting from the single side band mixing of saidmodulation carrier signal and said audio information signals; andwherein said processing means includes first phase shifting means forphase shifting said modulation carrier signal by 90°, second phaseshifting means for phase shifting said audio information signals by 90°,first double side band mixing means coupled to said first phase shiftingmeans for mixing said audio information signals and the 90° phaseshifted version of the modulation carrier signal, second double sideband mixing means coupled to said second phase shifting means for mixingthe 90° phase shifted version of the audio information signals with themodulation carrier signal, and means coupled to said first and seconddouble side band mixing means for summing the output signals therefromthe provide the desired scrambled audio signals.
 31. A system fordescrambling audio information signals previously scrambled bygenerating a scrambling modulation carrier signal at a frequency lyingwithin the frequency spectral range of original audio informationsignals of about 50 Hz to about 15 kHz and single side band modulatingthe original audio information signals with the modulation carriersignal to produce scrambled audio signals containing a frequencytranslated version of the original audio information signals, saidsystem comprising:means for generating a descrambling modulation carriersignal having substantially the same frequency as the scramblingmodulation carrier signal; and single side band processing means havinga first input coupled to said generating means and a second input forreceiving said scrambled audio signals for removing the scramblingsingle side band signal component therefrom to recover said originalaudio information signals; wherein the scrambled audio signals have avariable frequency component contributed by the scrambling modulationcarrier signal, and wherein said generating means includes means forvarying the frequency of the descrambling modulation carrier signal in asubstantially identical manner to the variations in the scrambled audiosignals; and wherein said scrambled audio signals include associatedfirst control signals specifying the start of frequency varyingoperations and second associated control signals specifying thefrequency varying rate; and wherein said generating means includes meansresponsive to said first control signals for initiating said varyingmeans and means responsive to said second control signals forestablishing the varying rate.
 32. A system for descrambling audioinformation signals previously scrambled by generating a scramblingmodulation carrier signal at a frequency lying within the frequencyspectral range of original audio information signals of about 50 Hz toabout 15 kHz and single side band modulating the original audioinformation signals with the modulation carrier signal to producescrambled audio signals containing a frequency translated version of theoriginal audio information signals, said system comprising:means forgenerating a descrambling modulation carrier signal having substantiallythe same frequency as the scrambling modulation carrier signal; andsingle side band processing means having a first input coupled to saidgenerating means and a second input for receiving said scrambled audiosignals for removing the scrambling single side band signal componenttherefrom to recover said original audio information signals; andwherein said processing means includes first phase shifting means forphase shifting the scrambled audio signals by 90°, first double sideband mixing means for mixing the scrambled audio signals with thedescrambling modulation carrier signal, second double side band mixingmeans coupled to said first and second phase shifting means for mixingthe 90° phase shifted version of the descrambling modulation carriersignal, and means coupled to said first and second double side bandmixing means for summing the output signals therefrom to remove thescrambling single side band signal component.